The Aztec calendar is the calendar system that was used by the Aztecs as well as other Pre-Columbian peoples of central Mexico. It is one of the Mesoamerican calendars, sharing the basic structure of calendars from throughout ancient Mesoamerica.
The calendar consisted of a 365 day calendar cycle called xiuhpohualli (year count) and a 260 day ritual cycle called tonalpohualli (day count). These two cycles together formed a 52 year “century,” sometimes called the “Calendar Round.” The xiuhpohualli is considered to be the agricultural calendar, since it is based on the sun, and the tonalpohualli is considered to be the sacred calendar.
The calendric year began with the first appearance of the Pleiades asterism in the east immediately before the dawn light.
The tonalpohualli (“day count”) consists of a cycle of 260 days, each day signified by a combination of a number from one to thirteen, and one of the twenty day signs. With each new day, both the number and day sign would be incremented: 1 Crocodile is followed by 2 Wind, 3 House, 4 Lizard, and so forth up to 13 Reed, after which the cycle of numbers would restart (though the twenty day signs had not yet been exhausted) giving 1 Jaguar. The cycle of day signs would continue until 7 Flower, after which it would restart and give 8 Crocodile. It would take a full 260 days (13×20) for the two cycles of twenty day signs and thirteen numbers to realign and repeat the combination 1 Crocodile.